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INTRODUCTION 361<br />

2.4<br />

2.0<br />

GaP<br />

AlAs<br />

Γ<br />

X<br />

L<br />

Band gap<br />

[eV]<br />

1.6<br />

1.2<br />

Si<br />

GaAs<br />

InP<br />

AlSb<br />

0.8<br />

Ge<br />

GaSb<br />

0.4<br />

InAs<br />

0.0<br />

5.4<br />

5.6<br />

5.8<br />

Lattice constant<br />

[Å]<br />

6.0<br />

6.2<br />

Figure 9.2<br />

their alloys<br />

Estimated band gap as a function of lattice constant for Si, Ge, III-V binaries and<br />

Efficiency<br />

[%]<br />

40<br />

30<br />

20<br />

10<br />

0<br />

Two-junction<br />

practical limit<br />

NREL Invention<br />

of GaInP/GaAs<br />

solar cell<br />

1985<br />

Patent<br />

issued<br />

1990<br />

Commercial<br />

production of<br />

tandem<br />

Year<br />

Three-junction concentrator<br />

Tandempowered<br />

satellite<br />

flown<br />

1995<br />

Production<br />

levels reach<br />

300 kW y −1<br />

2000<br />

Figure 9.3 These GaInP/GaAs cell efficiencies were measured at one sun with the AM1.5<br />

global spectrum. The triangles were measured under concentrated sunlight for three-junction<br />

GaInP/GaAs/Ge cells<br />

than 27% one-sun air mass 1.5 global (AM1.5G) were achieved by changing the top-cell<br />

thickness to achieve current matching [6, 7]. This tuning of the top-cell thickness can also<br />

be used to achieve current matching under different solar spectra, for example, AM0 and<br />

AM1.5direct (AM1.5D). Using this feature of the GaInP/GaAs tandem solar cell, NREL,<br />

over the next three years, set records at AM1.5G with an efficiency η = 29.5% [8], at<br />

160-suns AM1.5D with η = 30.2%, [9] and at one-sun AM0 with η = 25.7% [10]. Soon,<br />

numerous laboratories around the world were studying this device, and the 29.5% record<br />

was eventually eclipsed by researchers at the Japan Energy Corporation with an efficiency

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